9.2.10 Session hold initiated from CS network

29.1633GPPInterworking between the IP Multimedia (IM) Core Network (CN) subsystem and Circuit Switched (CS) networksTS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

When an MGCF receives a CPG message with a "remote hold" Generic notification indicator (signal 1 of figure 51), the MGCF forwards the hold request by sending an UPDATE or re-INVITE message containing SDP with "sendonly" or "inactive" media (signal 4 of figure 51).

When an MGCF receives a CPG message with a "remote retrieval" Generic notification indicator (signal 6 of figure 51), the MGCF forwards the resume request by sending an UPDATE or re-INVITE message containing SDP with "sendrecv" or "recvonly" media (signal 9 of figure 51).

If the MGCF receives a CPG with "remote hold" or "remote retrieval" before answer, it shall forward the request using an UPDATE message. If the MGCF receives a CPG with "remote hold" or "remote retrieval" after answer, it should forward the request using re-INVITE but may use UPDATE.

If link aliveness information is required at the IM-MGW while the media are on hold, and RTCP was previously disabled the MGCF should provide to the modified SDP RR and RS bandwidth modifiers specified in IETF RFC 3556 [59] within the SDP offers in the UPDATE or re-INVITE messages holding and retrieving the media as detailed in clause 7.3.1 of 3GPP TS 26.114 [104]. If no link aliveness information is required at the IM-MGW or RTCP was previously enabled, the MGCF should provide the SDP RR and RS bandwidth modifiers previously used.

The interworking does not impact the user plane, unless the MGCF:

– provides modified SDP RR and RS bandwidth modifiers in the UPDATE or re-INVITE messages. If the MGCF provides modified SDP RR and RS bandwidth modifiers in the UPDATE or re-INVITE messages, the MGCF shall also provide modified SDP RR and RS bandwidths to the IM-MGW using the Configure IMS Resources procedures (signals 2-3 and 7-8 of figure 51); or

– has identified a speech call as an "ICS call" as specified in clause 7.2.3.1.2.12 or in clause 7.2.3.2.7a. When the MGCF receives from the CS network:

a) a hold request the MGCF shall request the IM-MGW to suspend sending media towards the IMS side by changing the through-connection of the IM CN subsystem side termination to "not through-connected" (not depicted in figure 51, but may be combined with signal 2) prior to sending of the UPDATE or re-INVITE message containing the SDP offer with "sendonly" or "inactive" media;

b) a request to retrieve the session on hold and if the MGCF requested the IM-MGW to suspend sending media on the hold request the MGCF shall request the IM-MGW to re-establish communication towards the IMS network by changing the through-connection of the IM CN subsystem side termination to "both-way through-connected" (not depicted in figure 51, but may be combined with signal 7) prior to sending of the UPDATE or re-INVITE message containing the SDP offer with "sendrecv" or "recvonly" media; or

– has received for a confirmed dialog towards IMS side the Contact header (in the initial INVITE request or in the 200 OK (INVITE) response) with the "isfocus" media feature tag, defined in IETF RFC 3840 [135]. When the MGCF receives from the CS network:

Message sequence chart

Figure 51 shows the message sequence chart for the call hold and retrieval procedures.

Figure 51: Session hold/resume from CS network